Architecture for an acoustic liner

ABSTRACT

An acoustic liner  20  includes a remote panel  26 , a perforated proximate panel  22  transversely spaced from the remote panel and one or more partitions  32  spanning across the space between the panels. At least one baffle  44  cooperates with the partitions to define a resonator chamber  34   b  and an associated neck  56  for establishing communication between the resonator chamber and a fluid stream G flowing past the proximate panel.

STATEMENT OF GOVERNMENT INTEREST

This invention was made under U.S. Government Contract F33657-99-D-2051.The Government has certain rights in the invention.

CROSS REFERENCE TO RELATED APPLICATIONS

This application includes subject matter in common with co-pending,concurrently filed application entitled “Acoustic Liner with BypassCooling”, both applications being assigned to or under obligation ofassignment to United Technologies Corporation.

TECHNICAL FIELD

This invention relates to noise attenuating acoustic liners andparticularly to a liner having simple, cost effective features forattenuating low frequency noise and/or in which coolant bypasses theliner's acoustic resonator chambers to improve the durability of theliner without impairing its noise attenuating properties.

BACKGROUND

Acoustic liners are used in fluid handling ducts to attenuateundesirable noise associated with a stream of fluid flowing through theduct. Examples of such ducts include the inlet and exhaust system ductsof gas turbine aircraft engines. A typical acoustic liner includes aback sheet and a face sheet spaced from the back sheet to define aninter-sheet compartment. The liner is positioned along the duct wallwith the face sheet extending approximately parallel to the direction offluid flow through the duct. The inter-sheet compartment serves as aresonator chamber for attenuating noise. Walls may extend between thesheets to subdivide the compartment into multiple smaller resonatorchambers. A set of holes or necks perforates the face sheet to establishcommunication between the chamber (or chambers) and the fluid stream.One well known relationship that describes the noise frequency that aresonator will attenuate is:f=(c/2π)[A _(N)/(V _(C) L _(N))]^(0.5)where c is the local speed of sound, A_(N) is the cross sectional areaof the neck leading to a chamber (or the aggregate area of multiplenecks leading to a chamber) V_(C) is the volume of the chamber and L_(N)is the length of the neck. In a typical turbine engine acoustic liner,the face sheet is made of sheet stock. Accordingly, L_(N) is thethickness of the sheet.

When the acoustic liner is used in a hot environment, for example toline an afterburner duct, it may be desirable to cool the liner toextend its useful life. Cooling may be accomplished by supplyingcoolant, usually relatively cool air, to the resonator chambers andallowing the coolant to flow through the chambers and discharge throughthe resonator necks. However at the flow rates typically required forsatisfactory cooling, the coolant discharging through the resonatornecks degrades the liner's acoustic admittance, which is a measure ofits ability to admit acoustic disturbances into the resonator chambers.

Even if cooling is not required, it may be necessary to tune a resonatorchamber to attenuate low frequency noise. A resonator can be tuned to alow frequency by increasing V_(C) or L_(N) or by decreasing A_(N). Foraircraft engine applications, increasing V_(C) may not be an option dueto space constraints. Decreasing A_(N) also may not always be an option.This is partly because decreasing A_(N) reduces the overall porosity ofthe liner, which directly diminishes the acoustic admittance. Moreover,if the liner is cooled, the resonator necks also serve as coolantpassages to cool the face sheet. Decreasing the size or quantity of theresonator necks/coolant passages could therefore compromise thedurability of the liner. Accordingly, increasing L_(N) may be the mostviable option for tuning a resonator chamber to attenuate low frequencynoise.

SUMMARY

One embodiment of the acoustic liner described herein includes a remotepanel, a proximate panel transversely spaced from the remote panel and aresonator chamber residing between the panels. Perforations penetratethe proximate panel in registration with the resonator chamber. A neckwith an inlet recessed from the proximate panel establishescommunication between the chamber and a fluid stream flowing past theproximate panel. A bypass coolant passage guides coolant to theperforations without guiding it through the resonator chamber or theneck.

Another embodiment of the acoustic liner includes a remote panel, aperforated proximate panel transversely spaced from the remote panel andone or more partitions spanning across the space between the panels. Atleast one baffle cooperates with the partitions to define a resonatorchamber and an associated neck for establishing communication betweenthe resonator chamber and a fluid stream flowing past the proximatepanel.

The foregoing and other features of the various embodiments of theacoustic liner will become more apparent from the following detaileddescription and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional side elevation view of a conventionalafterburner screech liner for a gas turbine engine.

FIG. 2 is a schematic view similar to FIG. 1 taken in the direction 2-2of FIG. 3 and showing a liner with an innovative resonator chamber andan innovative bypass cooling system.

FIG. 3 is a developed view in the direction 3-3 of FIG. 2.

FIG. 4 is a view similar to FIG. 2 and taken along the section line 4-4of FIG. 5, 6, 7 or 8.

FIG. 5 is a developed view in the direction 5-5 of FIG. 4 showing aliner with axially distinct, circumferentially continuous resonatorbands.

FIG. 6 is a view similar to FIG. 5 but showing two classes of resonatorchambers in some of the resonator bands with similar classes of chambersbeing circumferentially aligned in successive multi-class bands.

FIG. 7 is a view similar to FIG. 6 but showing similar classes ofchambers being circumferentially misaligned in successive multi-classbands.

FIG. 8 is a view similar to FIG. 6 but showing similar classes ofchambers being circumferentially offset in successive multi-class bands.

FIG. 9 is a view similar to FIG. 6 and taken in the direction 9-9 ofFIG. 10 showing an alternate orientation of a resonator neck.

FIG. 10 is a view in the direction 10-10 of FIG. 9.

FIG. 11 is a view similar to FIG. 6 but showing an isolated resonatorneck.

FIG. 12 is a perspective view of a resonator chamber of FIG. 11.

FIG. 13 is a view similar to FIG. 4 showing a folded resonator neck.

DETAILED DESCRIPTION

FIG. 1 shows a conventional noise attenuating liner 20 used in anafterburner duct of an aircraft turbine engine. The liner is also knownas a screech liner. The illustration also indicates a coordinate systemwith axial, radial and circumferential (perpendicular to the plane ofthe illustration) directional components since such a system is anatural choice when describing turbine engines. However in otherapplications, including those not involving ducts, a more generalcoordinate system employing longitudinal, transverse and lateraldirectional components may be more appropriate. As seen in theillustrations, the longitudinal direction corresponds to the axialdirection, the transverse direction-corresponds to the radial directionand the lateral direction corresponds to the circumferential direction.

The acoustic liner 20 includes a proximate panel 22 circumscribing aduct centerline 24 and a remote panel 26 radially spaced from theproximate panel and also circumscribing the centerline to define aninter-panel annulus. The proximate and remote panels are also referredto as a face sheet and a back sheet respectively. Endwalls 28 join thepanels together at their axial extremities. Stiffeners 32, which extendcircumferentially 360 degrees, are attached to the panels and spanradially across the inter-panel space. The stiffeners act as partitionsthat cooperate with the panels to define inter-panel compartments 34.During engine operation a stream of hot gases G flows axially throughthe duct. The proximate panel is relatively hot due to its proximity tothe hot gas stream. The remote panel is relatively cold due to itsremoteness from the hot gas stream.

FIGS. 2 and 3 are more schematic views depicting the partitions 32 asstraight, radially extending elements and showing additional details ofan innovative liner. The proximate panel 22 includes perforations 36 inregistration with selected compartments to define resonator chamberssuch as representative chamber 34 a. Note that FIG. 3 and succeedingdeveloped views show only clusters of perforations 36 even though theproximate panel is perforated throughout. The perforations 36 serve asnecks for establishing communication between the chamber 34 a and thegaseous fluid stream G. The length of each neck equals the radialthickness t of the proximate panel. Since the proximate panel is usuallyrelatively thin sheet metal or other sheet stock, the neck length issmall. As a result, each chamber 34 a is effective at attenuatingrelatively high frequency noise. The perforations 36 form a perforationarray of relatively low porosity in comparison to the array ofperforations 38 discussed below. Coolant admission holes 42 maypenetrate the remote panel to supply coolant to the chamber 34 a. Theperforations 36 would then also serve as coolant discharge passages.However as noted above, the flow of coolant causes impaired acousticadmittance at the coolant flow rates required for satisfactory cooling.

Compartments 34 b include a baffle 44. The baffle of FIGS. 2 and 3 isformed of two opposing sheet metal pieces 46 c, 46 d. Each piece 46 c,46 d extends circumferentially 360 degrees, or may be made of individualsegments secured together to extend 360 degrees. Each piece 46 c, 46 dhas an axially extending leg 48 in contact with one of the partitions 32and a radially extending leg 50. The axially extending legs define abaffle base 54 while the radially extending legs define a resonator neck56 having a length L. The neck has an inlet 58 and an outlet 60. Theinlet is radially recessed by a recess distance R from the proximatepanel so that the baffle base 54, the proximate panel 22, and twoaxially neighboring partitions 32 define a coolant plenum 64. The bafflealso cooperates with the neighboring partitions 32 to define a resonatorchamber 34 b. Because the baffle extends 360 degrees, the neck 56 is acircumferentially continuous slot. The neck has a circumferentialdimension a and an axial dimension b both measured at the radiallocation of the inlet 58. In the liner of FIGS. 2 and 3, thecircumferential dimension exceeds the axial dimension. The proximatepanel 22 also includes the perforations 38 in registration with thecompartments 34 b. Perforations 38 form a perforation array of highenough porosity that the neck 56, not the perforations 38, influence thenoise frequency that the chamber responds to. Because the length L ofneck 56 significantly exceeds the length t of necks 36, chamber 34 bresponds to lower frequencies while chamber 34 a responds to higherfrequencies.

A bypass cooling system is provided to guide coolant to the perforations38 without requiring that the coolant first pass through the resonatorchamber 34 b or neck 56. The bypass cooling system includes radiallyextending bypass coolant passages 66 alongside the chamber 34 b and thecoolant plenum 64. Each passage 66 is defined by a passage wall 68cooperating with a partition 32. The passage may be circumferentiallycontinuous as shown in FIGS. 2 and 3, or may be circumferentiallysegmented into multiple sub-passages. A coolant intake comprising one ormore openings 70 in the remote panel admits coolant to the passage. Acoolant outlet 72 adjacent to the coolant plenum discharges the coolantinto the plenum. The coolant then flows through the perforations 38 tocool the proximate panel. Because the coolant bypasses the chamber 34 band neck 56, and because the high porosity array of perforations 38 havelittle or no effect on the acoustic admittance of chamber 34 b, the flowof coolant does not adversely affect the noise attenuating properties ofthe chamber 34 b. If necessary, optional coolant admission holes 76 maypenetrate the remote panel to supply additional coolant to chamber 34 b.Although the flow of this additional coolant through the resonator neck56 would degrade the acoustic admittance, the effect will be less severethan if all the coolant flowed through the chamber and neck.

Although the foregoing discussion describes the bypass cooling system inconjunction with an architecture for a low frequency resonator chamber,the two concepts may be used independently.

FIGS. 4-8 show various distributions of resonator chambers, which willnow be described in more detail. For simplicity, the figures show, andthe discussion describes, resonator chambers without the bypass coolingsystem. However it is clear that the chamber distributions describedbelow can be used with or without the bypass cooling system.

As seen in FIGS. 4 and 5 the resonator chambers 34 a formcircumferentially extending resonator bands 78 responsive to higherfrequencies. The resonator chambers 34 b form circumferentiallyextending resonator bands 80 responsive to lower frequencies. The lowerfrequency resonator bands 80 are axially offset from the higherfrequency bands 78. The presence of the higher and lower frequency bandscauses the liner to attenuate a broader spectrum of noise frequencies.

FIG. 6 shows an acoustic liner similar to that of FIG. 5 but includingauxiliary partitions 82 extending axially between neighboring partitions32 and radially between the proximate and remote panels 22, 26. Bafflepieces 46 c, 46 d extend circumferentially between selected neighboringpairs of auxiliary partitions. The baffles cooperate with the auxiliarypartitions 82 and primary partitions 32 to define a lower frequencyresonator chamber 34 b and its associated neck 56. The neck has acircumferential dimension a and an axial dimension b measured at thesame radius as the circumferential dimension. The circumferentialdimension exceeds the axial dimension. In FIG. 6, axially forward andaft resonator bands 84 each include a first class of circumferentiallydistributed resonator chambers 34 b defined in part by the baffles and asecond class of circumferentially distributed chambers 34 a withoutbaffles. The chambers 34 b are circumferentially interposed betweenchambers 34 a to form a resonator band responsive to multiplefrequencies. Resonator bands 78 without baffles are axially interposedbetween bands 84. The classes of chambers 34 a, 34 b in successivemulti-frequency bands 84 are circumferentially aligned with each other.

FIG. 7 shows a liner similar to that of FIG. 6 except that the chambers34 a and 34 b in successive multi-frequency bands are circumferentiallynonaligned with each other.

FIG. 8 shows a liner similar to that of FIG. 6 except that the chambers34 a are circumferentially offset with the chambers 34 b in thesucceeding multi-frequency band, but not completely misaligned as inFIG. 7.

FIGS. 9 and 10 show a liner in which the baffles are arranged so thatneck circumferential dimension a is smaller than neck an axial dimensionb.

FIGS. 11 and 12 show a liner similar to that of FIG. 6 except that thediscrete necks 56 are isolated rather than extending to the auxiliarypartitions 82 or to the principal partitions 32. The illustrated neck iscylindrical, but may have some other curved or faceted shape when seenin plan view as in FIG. 11.

FIGS. 2-11 depict liners in which resonator bands with baffles areaxially separated by resonator bands without baffles. Such anarrangement may be especially practical when used in conjunction with abypass cooling system. In principle, however, the bands without bafflesneed not be present. In other words, the entire liner can be constructedwith bands like, for example, the bands 80 indicated in FIGS. 4 and 5or, for example, the bands 84 indicated in FIGS. 6-8.

FIGS. 6-11 depict liners in which selected resonator bands 84 include aclass of chambers with baffles and a class without baffles. Inprinciple, the chambers without baffles need not be present.

FIG. 13 shows a resonator chamber whose elongated neck 56 is a foldedneck defined in part by a skirt 86 that extends radially inwardly fromthe remote panel 26. The effective length of the folded neck is the sumof the individual lengths L₁ and L₂. Typically the area A₁ equals theannular area A₂. The folded configuration is applicable to thecircumferentially continuous neck (FIG. 3), the localized neck (e.g.FIGS. 6 and 9) and the isolated neck (FIG. 11).

Although the baffled chambers and the bypass cooling system can be usedindependently, certain advantages may be obtained by using both conceptsin a single liner. Referring back to FIGS. 2 and 3, the chambers 34 acan be tuned to higher frequencies, and the tuning can be spatiallyvaried throughout the liner if desired. Similarly, the chambers 34 b canbe tuned to lower frequencies, and the tuning of these chambers can alsobe spatially varied if desired. However the chambers 34 b can be tunedto particularly low frequencies due to the extended neck 56. The regionof the face sheet in the vicinity of chambers 34 b can be cooled asdescribed with the bypass cooling system without compromising theacoustic admittance of chambers 34 b. This cooling may be less effectivethan the cooling in the vicinity of chambers 34 a because of the highporosity of the array of perforations 38 in comparison to the lowerporosity of the array of perforations 36. However the less effectivecooling may be justifiable in return for the acoustic admittance.Moreover, the coolant discharging through perforations 36 will form acoolant film on the surface 88 of the proximate panel. This film canextend across the region of the panel penetrated by perforations 38 andhelp compensate for the reduced cooling effectiveness in that region.Thus, the designer can trade acoustic performance for coolingeffectiveness and vice versa. These considerations will help determinethe pattern of chambers (for example the different patterns seen inFIGS. 6-8).

Although this invention has been shown and described with reference to aspecific embodiment thereof, it will be understood by those skilled inthe art that various changes in form and detail may be made withoutdeparting from the invention as set forth in the accompanying claims.For example, although the liner has been described in the context of aturbine engine afterburner duct, it can be applied to other types ofducts or to surfaces other than duct walls.

1. An acoustic liner for attenuating noise in a substantiallylongitudinally flowing fluid stream, comprising: a remote panel; aperforated proximate panel transversely spaced from the remote panel;one or more partitions spanning across a space between the remote andperforated proximate panels and contacting the remote and perforatedproximate panels to define at least one inter-panel compartment; and atleast one baffle cooperating with the one or more partitions to defineat least one resonator chamber and an associated neck for establishingcommunication between the at least one resonator chamber and a fluidstream.
 2. The liner of claim 1 wherein the remote panel is a relativelycold panel and the perforated proximate panel is a relatively hot panel.3. The liner of claim 1 wherein the liner circumscribes an axis, alongitudinal direction is axial, a transverse direction is radial, and alateral direction mutually perpendicular to the longitudinal andtransverse directions is circumferential.
 4. The liner of claim 1wherein the neck has longitudinal and lateral dimensions, and thelateral dimension exceeds the longitudinal dimension.
 5. The liner ofclaim 1 comprising multiple resonator chambers all tuned to the samefrequency.
 6. The liner of claim 1 wherein perforations penetrate theperforated proximate panel in registration with the at least oneresonator chambers and the liner comprises a bypass cooling system forconveying bypass coolant to the perforations without guiding the bypasscoolant through the at least one resonator chambers.
 7. The liner ofclaim 6 wherein the at least one baffles includes a base transverselyrecessed from the perforated proximate panel to define a coolant plenum,and wherein a bypass coolant passage is defined at lest in part by apassage wall and a cooperating partition.
 8. An acoustic liner forattenuating noise in a substantially longitudinally flowing fluidstream, comprising: a remote panel; a perforated proximate paneltransversely spaced from the remote panel; one or more partitionsspanning across a space between the remote and perforated proximatepanels and cooperating with the remote and perforated proximate panelsto define at least one inter-panel compartment: at least one bafflecooperating with the one or more partitions to define at least oneresonator chamber and an associated neck for establishing communicationbetween the at least one resonator chamber and a fluid stream; andwherein perforations penetrate the perforated proximate panel inregistration with the at least one resonator chamber and the linercomprises a bypass cooling system for conveying bypass coolant to theperforations without guiding the bypass coolant through the at least oneresonator chamber; and wherein the at least one baffle includes a basetransversely recessed from the perforated proximate panel to define acoolant plenum, and wherein a bypass coolant passage is defined at leastin part by a passage wall and a cooperating partition; and wherein thebypass coolant passage includes a coolant intake comprising an openingin the remote panel and also includes a coolant outlet for dischargingcoolant into the coolant plenum.
 9. The liner of claim 1 including askirt for defining a folded neck.
 10. The liner of claim 9 wherein theskirt extends from the remote panel.
 11. The liner of claim 1 whereinthe at least one resonator chamber is a lower frequency resonatorchamber and the liner also includes at least one higher frequencyresonator chamber.
 12. The liner of claim 11 wherein the perforatedproximate panel includes regions of higher porosity in registration withlower frequency chambers and regions of lower porosity in registrationwith higher frequency chambers, and the liner also includes a bypasscooling system for conveying bypass coolant to higher porosity regionswithout admitting the bypass coolant to the resonator chambers.
 13. Theliner of claim 11 comprising multiple lower frequency resonator chambersall tuned to a common lower frequency and multiple higher frequencyresonator chambers all tuned to a common higher frequency.
 14. The linerof claim 11 wherein the remote panel includes coolant admission holesfor admitting coolant into higher frequency resonator chambers.
 15. Theliner of claim 1 wherein the remote panel includes auxiliary coolantadmission holes for admitting auxiliary coolant into the resonatorchambers.
 16. The liner of claim 1 wherein the neck is laterallycontinuous.
 17. The liner of claim 1 wherein the neck is isolated. 18.The liner of claim 17 wherein the neck is cylindrical.
 19. The liner ofclaim 12 including a resonator band responsive to a higher range offrequencies and a resonator band responsive to a lower range offrequencies axially offset from the higher range of frequencies.
 20. Theliner of claim 1 comprising a longitudinally forward band of resonatorchambers and a longitudinally aft band of resonator chambers, each bandincluding laterally distributed resonator chambers of at least twoclasses.
 21. The liner of claim 20 wherein the resonator chambers arelaterally distributed so that resonator chambers of a given class in theforward band are laterally aligned with resonator chambers of the sameclass in the aft band.
 22. The liner of claim 20 wherein the resonatorchambers are laterally distributed so that resonator chambers of a givenclass in the forward band are laterally nonaligned with resonatorchambers of the same class in the aft band.
 23. The liner of claim 22wherein the resonator chambers are laterally distributed so thatresonator chambers of a given class in the forward band are laterallyoffset relative to resonator chambers of the same class in the aft band.24. The liner of claim 1 wherein the perforated proximate panel includesat least a first set of perforations in communication with higherfrequency resonance chambers and at least a second set of perforationsin communication with lower frequency resonance chambers, the first andsecond sets of perforations having different porosities.
 25. The linerof claim 24 including a plenum positioned directly between the at leastone resonator chamber and the perforated proximate panel, such that theplenum is formed by an area defined between the one or more partitions,the at least one baffle, and the perforated proximate panel, and whereinthe associated neck has an inlet at one end defined in a baffle base ofthe at least one baffle and an outlet at an opposite end positionedwithin the at least one resonator chamber.
 26. The liner of claim 25wherein the at least one baffle includes first and second pieces, eachof the first and second pieces including an axially extending leg incontact with one partition to form the baffle base, and each of thefirst and second pieces including a radially extending leg that formsthe associated neck, and wherein the inlet is formed within the bafflebase such that the associated neck does not extend into the plenum.